Typically, a planetary gear train is realized by combining a plurality of planetary gear sets and friction members. It is well known that when a planetary gear train realizes a greater number of shift speeds, speed ratios of the planetary gear train can be more optimally designed, and therefore a vehicle can have economical fuel or gas mileage and better performance. For that reason, the planetary gear train that is able to realize the greater number of shift speeds is under continuous investigation.
Though achieving the same number of speeds, the planetary gear train has a different operating mechanism according to a connection between rotation elements (i.e., sun gear, planet carrier, and ring gear). In addition, the planetary gear train has different features such as durability, power delivery efficiency, and size, depending on the layout thereof. Therefore, designs for a combining structure of a gear train are also under continuous investigation.
In addition, the planetary gear train realizes a plurality of shift-speeds. However, another friction member must be operated after one friction member is released so as to shift to a neighboring shift-speed from a view of shift control. In addition, a step ratio between the neighboring shift-speeds should be controlled to be suitable according to the planetary gear train.
The above information disclosed in this Background section is only for enhanced understanding of the background of the inventive concept and therefore it may contain information that does not form the prior art.